1. Field of the Invention
The present invention relates to a novel generator for the reaction of carbon-containing raw materials and also to an improved process for the production of carbon monoxide gas (CO gas) having a high degree of purity using such a generator.
2. Description of the Prior Art
Carbon monoxide gas is frequently produced in the art by means of a continuous process in which carbon-containing raw materials are reacted with oxygen and carbon dioxide at relatively high temperatures using the Boudouard equilibrium.
The principle of a vertical shaft furnace for such thermal processes has been known for a long time from metallurgy and is described, for example, in “Lueger, Lexikon der Technik, Vol. 16 (1970), Verfahrenstechnik and in Vol. 5 (1970), Hüttentechnik”. However, it does not meet the demands of an efficient continuous CO gas production installation in many respects.
Accordingly, attempts have repeatedly been made in the past to improve the gasification of coal in various respects.
U.S. Pat. No. 3,635,672, as the closest prior art, describes a coal gasification process in a vertical reactor containing separating plates which are permeable to gas and solids and which separate the reaction chamber into solids regions and gas chambers.
CO2 gas and CO gas are introduced into the fluidised coke bed from beneath and O2 is introduced into the gas chambers from the side in order, by the combustion of CO to CO2, to generate the necessary heat energy required for the endothermic reaction of CO2 with C to form CO in the fixed bed.
This process has the disadvantage that the elements built into the reactor are very complex and make high demands of the flowability of the solid material in order to avoid blockages and hence impairment of the combustion process. In addition, they represent a cost factor in the construction and maintenance of the installation and reduce the space-time yield of the reactor to a not inconsiderable degree.
DE 34 26 912 describes a specific shaft furnace as the reactor and a process for the gasification of coke. In this specification, a CO gas having a purity of more than 90 vol. % is produced from coke using oxygen and carbon dioxide. Charging with coke takes place at the upper end of the shaft, where the CO gas produced is also discharged counter-currently. A combustion nozzle is arranged horizontally at the base of the shaft furnace at the slag outlet opening and conveys the combustion gases to the coke bed, the CO gas produced being mixed, as fuel, with oxygen and carbon dioxide before it enters the nozzle. In this manner, a flame forms at the nozzle, which should ensure that the slag flows away unhindered.
A disadvantage of this process is the formation of a flame at the burner and its predominantly horizontal orientation beneath the coke bed, with the result that only inadequate control of the coal gasification process is possible. This manifests itself, for example, in the CO gas purity that is achieved of only 92.5 vol. % and a proportion of 3.0 vol. % for each of hydrogen and carbon dioxide. A further disadvantage of this process is the addition of flux agents and the discharge of combustion residues in the form of liquid slag.
EP 142 097 describes a similar variant of such a CO gas generator, the nozzle for the gasification agents O2 and CO2 passing laterally through the jacket of the tubular shaft furnace and pointing downwards, thus facilitating the discharge of slag.
As our own tests have shown, such nozzle arrangements have the disadvantage that the combustion zone created inside the furnace is asymmetrical, which leads to overheating of the opposing side of the jacket of the tubular shaft furnace and which must be avoided at all costs in the case of steel jackets without additional heat-insulating lining.
GB 1 453 787 describes the gasification of coal in a shaft furnace which is likewise charged with coke from above, CO2 being injected from beneath and O2 being injected laterally through porous bricks, so that the combustion zone is created in the middle of the shaft furnace and the CO gas escapes at the top.
A disadvantage of this process is that the capacity of the shaft furnace is utilised wholly inadequately because coke that has not been burnt is removed at the base of the furnace and fed back into the system from the top, until the ash content has reached a critical limit. The patent contains no information regarding the purity of the CO gas.
U.S. Pat. No. 4,007,015 describes the production of CO gas in a two-chamber furnace having a heat exchanger, CO being obtained in a subsidiary process in addition to the CO2 production. The CO is obtained without supplying oxygen to provide additional energy for the reaction of CO2 with carbon, the CO2 gas entering the CO-producing chamber filled with coke being heated solely by a common heat exchanger.
This process has the disadvantage that possibilities for controlling the process in order to produce a qualitatively highly pure CO gas are insufficient.
NL 8 303 992 is to be regarded as more remote prior art, in which there is described a vertical shaft furnace with fire-resistant wall lining, in which carbon dust is burnt in a turbulent air stream, with the creation of a so-called “raceway” in which the gasification of coal takes place.
Other processes work with the aid of catalysts such as, for example, Cs2CO3 (U.S. Pat. No. 3,758,673) or cobalt oxide (U.S. Pat. No. 3,801,288).
There has also been no lack of attempts to improve the difficult process of discharging residues, predominantly in the form of liquid slag, from the shaft furnace during the gasification of coal. Examples thereof are given in patent specifications GB 1 098 552, GB 1 512 677 and DE 27 38 932. The major disadvantage of these processes is that the combustion residues do not occur in finely divided solid form which could be discharged with the flue dust, but must be discharged in the form of liquid slag, which is difficult to handle.
All these cited examples of the prior art exhibit deficiencies which are troublesome for a modern production operation from the point of view of environmental protection, operating safety and economic efficiency.
An object of the present invention was, therefore, to develop a novel shaft furnace (referred to as a generator hereinbelow) which creates and maintains a stable combustion zone during the whole of the operating time of the furnace and which accordingly ensures a uniform combustion process. Such a uniform combustion process is an important requirement for the observance of high purity criteria for the CO gas that is obtained. Furthermore, CO emissions are to be avoided, especially during the operation of charging the tubular shaft furnace. A further object of the invention was to remove sufficient dust (more than 95%, preferably more than 99%) from the CO gas that has been produced, before any subsequent working-up steps, such as, for example, catalytic desulfurisation, and to dispose of the solid flue ash fractions in an environmentally suitable manner.
A further object of the invention was to provide a continuous process for the production of CO gas by the gasification of coal using the generator according to the invention, which process does not exhibit the disadvantages described above. This means in particular avoiding the formation of liquid slag and the discharge thereof from the apparatus.
A further object of the invention was to produce a CO gas having a purity of greater than 96 vol. %, preferably from 97 to 98 vol. %. The CO gas should in particular contain not more than 1.5 vol. % hydrogen (preferably <1.2 vol. %, particularly preferably <0.7 vol. %), not more than 0.15 vol. % oxygen (preferably <0.10 vol. %, particularly preferably <0.08 vol. %) and not more than 50 ppm methane (preferably <35 ppm, particularly preferably <25 ppm). Depending on the residual sulfur content in the carbon raw material employed, which is dependent on the origin of the raw material, the CO gas produced therefrom also contains further amounts of up to 7000 mg/Nm3 (preferably <5000 mg/Nm3, particularly preferably <3000 mg/Nm3) of organic sulfur compounds and up to 500 mg/Nm3 of inorganic sulfur compounds (preferably <300 mg/Nm3, particularly preferably <200 mg/Nm3).
In this text, the term Nm3 is understood as meaning 1 m3 of a gas (e.g. CO, O2) at a temperature of 20° C. and a pressure of 1.01325 bar.